In an isotopic separation device, an isotopics substance contained in a vapor flow is selectively excited by photoionization, for example.
In known device for distributing laser beams used in isotopic separation reactions such as those described in the French application no. 8 504 827 of Mar. 29, 1985, the selective excitation and photoionization beams necessarily have different polarization in the irradiation zone; in the case where these polarizations are linear they are perpendicular, and are either right or left in the case of circular polarizations.
FIG. 1 illustrates a device according to the prior art. Excitation beam Se and photoionisation beam St, of perpendicular polarization, are superimposed and directed towards a reaction chamber 10 by means of a first Glan prism 15. The Se beam can itself contain several beams having several different wave lengths. The Se and St beams have different absorption cross sections : the Se beam is more absorbed than the St beam. After many passages inside the chamber, the Se beam is attenuated and the selective reaction process is no longer effective. The residual beam St is sent towards a second Glan prism 15 and for a second time is superimposed onto an Se beam coming from a laser source (not shown). After traversing the second Glan prism 15, the superimposed beams Se+St are sent to the chamber until the beam St is fully absorbed. According to the selected atomic transitions, the fact that the polarizations of beams are secured by the assembly is unfavorable.
Other devices dividing the beams by means of suitably-adapted partially reflecting mirrors have been studied in order to overcome this drawback -- there is an open choice for polarizations.
FIG. 2 shows such an arrangement of the device using partially reflecting mirrors. The selective excitation beam Se capable of containing several beams of different wave lengths is superimposed onto one photoionization beam St. The superimposed beams are divided into two parts Pl and P2 by a first partially reflecting mirror Ml. The Pl part is directed towards the reaction chamber 10. The part P2 is directed towards a second partially reflecting mirror by means of synchronization means 16. The part Pl is sent into the chamber 10 for multiple passages. When the fluence of the beam St is equal to the wavelength saturation fluence of this beam, i.e. when the number of pulsed photons per unit area on the photoionization wavelength is equal to the reciprocal value of the absorption cross section for this wavelength, the residual part of Pl is superimposed onto the part P2 coming from the partially reflecting mirror Ml. The pulses Pl and P2 are synchronized by synchronization means 16.
A second partially reflecting mirror M2 divides into two the beams Pl and P2 originating from the partially reflecting mirror M1.
The choice of the reflection coefficients of partially reflecting mirrors, the path length inside the chamber between the two mirrors and the number of mirrors makes it possible to use more advantageously the energy of the various laser beams.
In this type of device, efficiency is limited by the formation of interference between the residual part P1 and the part P2 of the beams during their superimposition. This interference destroys the spacial homogeneity of the beams and which is vital for proper working of the extraction process.